I'm with AT on this one. I wouldn't even bother with an official "break-up", just let the dying embers fizzle out. As for his guilty feelings, there's nothing "uncommitted" about leaving a relationship that isn't meeting your emotional needs. If he can't handle leaving it open, he can tell her that he needs more attention to be in this relationship, or else he needs to end it, because he can't be in this limbo situation. Then leave the ball in her court.
How does a wife help her husband cope with a failing relationship? The same way any best friend helps another one deal with, even when yours is going well. And it's even easier because your husband isn't losing all the love in his life, unlike a monogamous BFF in the same situation.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker